The great Kilimanjaro, is it a magnificent feature of Earth’s surface or of its atmosphere? Although the answer might sound obvious, it arguably depends on whether you are examining the vast surrounding plains or looking from the summit.
Image Credit: OTT HydroMet
No matter which way you look at it, fascinating insights have been offered into a unique climate-glacier relationship and east Africa’s tropical troposphere by meteorological measurements on the mountain at 5,775 m – exactly halfway through the atmosphere at 506 hPa.
With a vertical relief of 5,000 meters, Kilimanjaro encompasses more than 3,000 km2 of northern Tanzania. 50,000 people endure the crowds and expense each year - a fact immediately explained by spending time on the mountain.
Kilimanjaro, as many seem to know, is fascinating in numerous respects. Among the many incredible aspects, it boasts:
- A vibrant, adaptable human culture that has developed over millennia and in many respects remains intact today.
- The zonation of ecological assemblages, which often changes with every meter of ascent
- The dramatic diurnal cycles characterizing Kilimanjaro’s climate, which changes rapidly with elevation
- The welcoming, cheerful people working on the mountain, outnumbering tourists ~4:1
- The incredible panoramic view from high on the mountain, which looks down onto the cloudbank but also to lakes, distant peaks and human development below
It is a rare day that the mountain’s atmosphere is quiescent; convention above the slopes is driven by intense radiation, which expands during the day both spatially and vertically until airflow (from another direction) dries out the rising parcels. As can easily be discerned from the description, these natural processes are simply enchanting.
The enormous summit comprises a relatively flat caldera that measures over 2 km in diameter. There are multiple reminders that Kilimanjaro is, in fact, a volcano: not limited to the array of concentric ‘craters’; the elemental sulfur deposits, sulfur dioxide emissions and steam vents – all offering clear evidence that volcanic activity continues rather than having ceased beneath the surface.
Figure 1. Panorama across the summit caldera looking over the Furtwängler Glacier remnants to the Northern Ice Field (~2.3 km), from near Uhuru Peak. Image Credit: OTT HydroMet
At times, Hemingway’s description of Kilimanjaro as “unbelievably white in the sun” is not hyperbole. Amongst visitors and locals alike, there is little doubt he was referring to the summit glaciers: an ice cap’s ragged fringe that once encircled the wide caldera and trickled down onto the flanks.
It is clear that the ice-covered area must have been hugely diminished from the extent of its expanse in the late 19th century, but the glaciers remain starkly beautiful against the deep blue sky and dark volcanic ash beyond. The color of the glaciers changes with the lighting: when albedo is high and intense incoming solar radiation is reflected back, they become a brilliant white.
Over 45 meters thick in places, Kilimanjaro’s glaciers perfectly preserve a precise record of environmental change - which, it must be noted, does not exist in ice anywhere else on the African continent.
Lonnie Thompson (Ohio State University), in February 2000, undertook the challenge of drilling six ice cores at the summit of the mountain. From these, Thompson developed a record that may span as many as 12,000 years.
No systematic meteorological measurements existed from the summit at the time of drilling, with only a handful of anecdotal temperature measurements. All existing climate stations at the time were approximately 4,000 meters lower in elevation, and the data they collated revealed nothing about radiation, airflow, temperature or precipitation high on the mountain.
An automated weather station (AWS) was installed during drilling to provide a physical basis upon which to interpret the ice cores near the deepest site.
From a short-term experiment, the principle objective has expanded to an objective to comprehensively characterize the current summit climate, which incorporates considerable inter-annual variability.
When it comes to understanding the long-term history of Kilimanjaro’s glaciers, the ice-core records, and the larger-scale causal mechanisms driving environmental changes currently underway in east Africa, are proving valuable in facilitating understanding.
Figure 2. Automated weather stations (AWS) on Kilimanjaro’s Northern Ice Field. The original AWS installed in February 2000 is on the left, including an albedometer and pyrgeometer; another tower with a net radiometer and supplemental instrumentation was added in 2010 and 2012. Image Credit: OTT HydroMet
The glacier AWS has proven successful beyond all expectations after 14 years. Kilimanjaro provides a fixed platform for continuous measurement of the tropical troposphere, but it must be noted that maintaining the station on a constantly-changing glacier surface requires significant effort.
The AWS allows measurement of a broader suite of variables (e.g., radiation) and with a much higher frequency compared to transient radiosonde observations at this level.
Installed in 2000, the initial station included sensors to measure wind direction and speed, reflected and incoming solar radiation, downward longwave radiation, aspirated and naturally ventilated air temperature and humidity, surface temperature, snow accumulation and ablation (i.e., Ultrasonic distance) and barometric pressure (as evidenced in figure 2).
Argos satellite telemetry is used to transmit four-hourly values from most of these measurements from the station, which has performed flawlessly over the entire period.
Additional sensors were installed in 2010 and 2012, which are compatible with those of the USCRN (Climate Reference Network). The newer installations include a continuously-ventilated radiation shield which houses several PRT temperature sensors and a warmed, high-accuracy humidity sensor.
An infrared temperature transducer and a four-component, integrated net radiometer were also added. These new sensors are yielding a comprehensive new view of summit climate through improved measurement accuracy.
Figure 3. AWS on Kilimanjaro’s Northern Ice Field at 5,775 m, with net radiometer (left-hand side) and fan-aspirated shield for temperature & humidity sensor (right). Mt. Meru is 70 km distant in the background. Image Credit: OTT HydroMet
Figure 4. CNR 4 Net radiometer (Kipp & Zonen) and an infrared temperature transducer (Apogee) installed on Kilimanjaro in Oct. 2012, both of which view the same area of glacier surface. Image Credit: OTT HydroMet
A variety of factors have influenced the success of climate measurements on the Northern Ice Field. Some of these factors were anticipated, including the high elevation and the mountain’s free-standing nature, while others (like the following list) occurred by chance:
- Topography only minimally disrupts the airflow over the broad, dome-shaped glacier. To the east - the predominant wind direction - the ice surface and relatively-flat snow extend ~800 m from the AWS. Good ventilation of instruments is provided by wind speed averaging 6 m/second, which improves accuracy - but high wind loading on the tower is rare.
- Typically, air at Kilimanjaro’s summit is dry, with an average vapor pressure of ~ 2 hPa and average annual precipitation at under 300 mm. Stratiform and/or convective clouds often form around the mountain, sporadically reaching an even higher altitude, whilst a clear sky prevails over the summit caldera at the same time. This donut-shaped pattern is an important feature of summit climate, despite the fact it is not even visible from the surrounding plains. Rime ice development on instruments is infrequent due to the dry air and intense incoming solar radiation, as can be shown by time-lapse camera images. Any rime that does develop typically sublimates or falls off within hours to days, even during the two seasonal wet periods each year (e.g., figure 5, middle).
- Instrument accuracy is maintained between service intervals and calibrations because air quality on the Northern Ice Field is very high. A recent analysis of trace elements in one of the ice cores found very low concentrations of insoluble particles, despite the large area of exposed caldera.
- An abundant power resource is provided by intense solar radiation via photovoltaics.
- Almost exclusively, long-term station data in east Africa is from elevations at least 4,000 m lower than the Kilimanjaro AWS. This means it is beneath a persistent inversion layer - and is much less representative of both the free atmosphere and the summit climate.
- Beneath the AWS is a true environmental archive that has been preserved by the glacier, which is believed to have begun forming almost 12,000 years ago. Through an improved understanding of how the glacier records climate, modern climate measurements are assisting in the interpretation of the ice core record.
Figure 5. Three timelapse-camera views from Kilimanjaro’s Northern Ice Field, illustrating the variability of glacier surface texture and albedo; note ablation stake in foreground. Left to right: 12 October 2009, 11 January and 3 February 2010, all at 18:00 local time. Image Credit: OTT HydroMet
A particularly exciting development is the newest measurements from the recently-added net radiation sensor. Each radiation component was measured separately until this instrument was added because engineering and spatial considerations necessitate situating instruments close to the tower.
The tower adversely influenced measurements, being too prominently within the instruments’ fields of view. With the integrated and lightweight sensor, measurements are able to be made further from the tower and, therefore, better represent the four variables (as depicted in Figure 4).
In addition, far less time is consumed by any leveling adjustment during fieldwork at the site (5,775 m) as compared to that required in leveling four different instruments.
The sheer intensity of radiation on Kilimanjaro was demonstrated by the first year with an integrated net radiometer at the site: median incoming shortwave at mid-day – across all days of the year – was almost 90% of that at the top of the atmosphere.
There was a wide variation in net shortwave radiation due to the control of reflectivity by surface albedo, which spanned from 0.90 following snowfall to 0.31 during the dry season. Generally speaking, the measurements reveal a very close correspondence between the net solar radiation and the variability of snowfall/snow surface.
The sensitivity of net shortwave receipt to snowfall is demonstrated in Figure 6 (e.g., the late-September event), but this also underlines the significance of snowfall magnitude – as shown by the gradual increase after the March-May “long rains.”
During the extended dry season of June to September, albedo is gradually lowered as the snow ages, which results in close to a doubling of net shortwave radiation, and causes continuous ablation through the year’s coldest months (not shown).
Figure 6. Timeseries of glacier surface height and net shortwave radiation on the Northern Ice Field (NIF), with both shown as 7-day running averages. Datum for surface height is 25 Feb. 2000. Note reversed y-axis scale for radiation. Image Credit: OTT HydroMet
It is therefore confirmed, by these new radiation measurements, that the variability of net radiation is the most controlling factor in the “health” of these glaciers.
Modeling with collaborators at the University of Innsbruck (Austria), along with earlier measurements, demonstrated the sensitivity of Kilimanjaro glaciers to the variability of snowfall amount and its timing.
An obvious reason for this sensitivity is that snowfall adds mass to the glaciers; perhaps more importantly, however, it is through albedo, or the governance of surface brightness, that the extent to which solar radiation is reflected rather than absorbed is controlled (figure 5).
This energy is available to drive melt and sublimation when radiation is absorbed at the glacier surface, both of which remove mass from the glaciers and cause shrinkage.
Many tropical mountains support a high degree of biodiversity despite the fact that details of high-elevation climate are poorly documented. Some ecological communities will, however, be unable to migrate upward as fast as the troposphere warms.
There is, therefore, an opportunity afforded by Kilimanjaro to make a full suite of climate measurements, operating high above any others existing in the region and thus providing a necessary and significant tool in the assessment model performance - as well as measurements made over a larger spatial area by satellites, radiosondes and Reanalysis data.
Hopefully, Kilimanjaro measurements will be able to continue until inter-annual variability is better resolved, and collaborative investigations will continue and develop in the meantime.
This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.
For more information on this source, please visit OTT HydroMet.